The present invention relates to a method of biological purification of sewage in a purification plant comprising three tanks only, of which at least two sequentially serve as a feeding tank for the sewage and at least one as a clarification tank, and wherein the sewage at any time is subjected to an aerobic treatment in the plant by means of aeration and simultaneous circulation of the sewage.
Such method may be carried out in two different ways. In the first instance, one and the same tank is always used as clarification tank and the two other tanks are sequentially used as feeding tanks, and in the last mentioned tanks, the sewage is subjected to an aeorbic treatment by adding air to the sewage and circulating the sewage. A rather simple plant may be used for carrying out this method due to the fact that only two of the tanks have to be provided with the mechanical means necessary for adding the air and for circulating the sewage.
According to the other known method, a plant is used wherein all three tanks are provided with means for adding air to the sewage and for circulating the sewage, and the three tanks are used in accordance to a predetermined scheme sequentially as clarification tank, as feeding tank for the sewage, and as aerobically working tanks.
By means of the last mentioned method, organic materials are removed from the sewage because the organic materials are decomposed and deposited as sludge in the clarification tank from which the sludge may be returned to the aeration tanks and surplus sludge, if any, may be removed.
After such treatment the sewage still contains inorganic compounds.
It is the object of the present invention to provide a method of the former kind by means of which the sewage may be purified for inorganic compounds also, in particular nitrogen compounds.
In order to purify sewage for both organic materials and nitrogen compounds, it is known to treat the sewage in the following way:
During a first step, the sewage is subjected to an aerobic treatment in an aeration tank and is then passed to an after-clarification tank. The sluge from the after-clarification tank is returned to the aeration tank, and the water from the after-clarification tank is passed to another aerobically working aeration tank wherein the nitrogen compounds are oxidized into nitrate (nitrification) by the action from microorganisms. After such aeration, the water is fed to an after-clarification tank and therefrom to a third tank wherein the water is subjected to an anaerobic treatment during which a nourishing agent for micro-organisms is added so that such microorgansims are able to reduce the nitrate into free nitrogen (denitrification) which escapes from the water. From the last mentioned tank the sewage is transferred to an after-clarification tank from which the purified sewage is removed. The sludge from the latter after-clarification tank is returned to the denitrification tank.
This dividing into three steps is made in order to secure that the correct microorganisms work in each of the steps viz. in the first step for decomposing organic material, in the second step for nitrification and in the third step for denitrification. Accordingly, this known method requires three tanks each followed by an after-clarification tank, and the sewage must pass these six tanks in series.
An attempt has been made for simplifying such plants by making a pilot-plant (Wuhrmann) wherein the sewage is fed to a first tank wherein the sewage is treated aerobically in order to achieve nitrification. Then the sewage is passed to another tank wherein it is treated anaerobically in order to reduce the nitrate produced in the first tank into free nitrogen. The sludge contained in the second tank is mineralized in order to produce the energy necessary for the denitrification. However, only under specific circumstances, this simplified method has worked successfully presumably because the sole source of energy for the denitrification is the mineralization of the sludge. However, the plant is simple due to the fact that only three tanks are necessary, arranged after each other, of which the last tank works as an after-clarification tank from which the sludge deposited is returned to the first tank.